Mmm where to start. Preformance, I'd started playing CS 5 years ago with a good frame rate. As more and more updates came before and after steam the FPS kept slowly dropping. ATM I run a machine 2 - 4 times (depending on which part) more powerful than when I started to play and I don't even get the same frame rate as I had back then. So 2* + increase in requirements to play basically the same game WTF.
Server preformance is another huge bug bear, having played on the same servers over that time I've seen the latency's from the servers go up and up as the FPS of the servers went down (any idea how fun it is to play on a server running at 1 FPS or under is ?), again playing the same game GG.
CS:S is a minefield all unto itself. 1 fps on a P4 2.4 with a 9800pro is wonderful. Let alone the slide show (under 30 fps) that almost always happens firing at an enemy, just when you really need the best FPS out of the game.
Cheating, is another huge thing Valve almost completely disregards, the most timely update I'd ever seen from them was 4+ years ago now when the first speed hacks showed up, and that still took a good week coming. You'd think it would be in their best interest to put resources into catching and banning these miscreants ASAP, to keep the honest players happy and get more money from the twits as they go out and buy yet another copy of the game (as does happen, having seen a number of people have 10+ cd keys banned from our servers having been caught by the admins or the handful of others that also waste their time trying to keep the game clean for Valve to suck people in). Add to that the 100+ variables that are left in that if there is a chance some can be exploited for an advantage over other players you can guarantee that it's being done regularly. Why the hell do players need access to 99% of them anyway.
Now onto the great protection steam is for Valve and how they protect it. Recently there had been a way for people to generate new Steam_ID's for themselves (creating new accounts), mmm great for end users when you want to ban someone over 100 times for cheating within the space of a couple of weeks. One of the fools using this, ISP's was contacted to do something about him which they didn't. So the whole ISP are now banned from playing on our servers affecting 10 - 15 people, great for customer relations for everyone. While Valve took their sweet assed time to fix the problem over months, all the while losing revenue.
How about half finished product release with CS:S lack of skins, maps (although this was never to be a big problem with the CS mapping community being large) to bugs and commands not working. You can tell I enjoyed that time I wasted when someone blocked a choke point and I couldn't kick or ban them from the console. Instead having the go to our external program and placing a temp ban on him there, which we would never have needed if the servers would keep the 1000's of bans in place all the time. Thanks Valve.
Next the faults with CS.
CS Report , here is just some of the odd things that happen. I've seen other complaints come up with video footage of other bugs. Plus the many I've picked up on over the years, all can make for a shocking experience. A number of them have never been fixed over the years. They happen online or offline, verse bots even. And don't seem to be fixed to any one hardware or driver config having run many drivers, a few graphics cards, cpu's, motherboards Etc. Alot of which started when CS went commercial and Valve moved to their own code.
Or how about the steam content servers only running on Windows 2003 with ads popping up from the content supplier even if you were connecting to a competing ISP's servers from their service (even if there was a way to tell the clients where to download the content from some ISP's only run Linux). Which at least 1 large ISP in Australia was annoyed with saying they were going to take down their 50+ Half-Life based servers.
And I'm only scratching the surface here.